1. Field of the Invention
The invention relates generally to the field of electromagnetic induction logging of Earth formations penetrated by a wellbore. More specifically, the invention is related to methods for adjusting measurements made by an electromagnetic induction well logging instrument for the effects of fluids (or air) in the wellbore and the conductivity of formations proximate the wall of the wellbore. An iterative method is described to correct the induction measurements for the effect of the wellbore, taking into account the conductivity of the formation immediately surrounding the wellbore.
2. Background Art
Electromagnetic induction well logging instruments are used to determine electrical conductivity of Earth formations penetrated by a wellbore. The electrical conductivity of formations is used, for example, to infer the presence of hydrocarbons in certain formations. A typical induction well logging instrument includes a generally elongated cylindrical sonde configured to move along the interior of the wellbore. The instrument sonde includes one or more transmitters, usually in the form of wire coils, and a plurality of receivers, also usually in the form of wire coils, the receivers being spaced at different selected longitudinal distances from the transmitter(s) along the mandrel. Circuitry in the instrument is used to generate electric current to energize the transmitter(s) and to detect various attributes of signals detected by the receivers. Electric current is passed through the transmitter(s) to induce electromagnetic fields in the formations surrounding the wellbore. Voltages are induced in the receiver as a result of the currents induced in the formation. Certain components of the induced voltages are related to the conductivity of the media surrounding the instrument.
In order to determine formation conductivities more accurately, it is useful to be able to determine the effect of any materials (mud or air) in the wellbore itself on the measurements made by the various receivers in the instrument. Taking such effects into account and adjusting the measurements made by the instrument is referred to as “borehole correction.”
When performing borehole correction for the conductivities measured by earlier, analog version induction instruments, such as one marketed under the trademark DIT by affiliates of the assignee of the present invention, the spatial distribution of the induction response, called “pseudo-geometrical factor” was considered to be independent of the conductivities of the various surrounding media, and the borehole-corrected conductivities were obtained from resulting linear equations, separately, for each of the medium induction response (ILM) and the deep induction response (ILD). For details, see Schlumberger Log Interpretation Principles/Applications Schlumberger Educational Services (1989).
The borehole correction procedure for another series of instruments, marketed under the trademark AIT by affiliates of the assignee of the present invention, is based on a true inversion with respect to some of the relevant parameters. Such an inversion is possible when measurements from several “short induction arrays” (arrays being induction receivers including a main receiver coil and a series connected, inverse polarity “bucking” coil both spaced closely to the induction transmitter) are available. For example, assuming that the wellbore diameter and the wellbore fluid (“mud”) conductivity are known from other measurements (e.g. a caliper and a mud resistivity sensor), it was possible to perform inversion with respect to formation conductivity and the tool standoff. For a more detailed description of such borehole correction procedures, see, e.g., U.S. Pat. No. 5,041,975, issued to Minerbo et al. and assigned to the assignee of the present invention.
The principles of a borehole correction procedure for a three-dimensional induction instrument, marketed by affiliates of the assignee of the present invention under the trademark RTSCANNER, are similar conceptually to the above procedures described in the '975 patent, but the three-dimensional procedure itself is much more complicated because nine components of a three-dimensional conductivity tensor are involved. For details of the foregoing procedure, see U.S. patent application Publication No. 2005/0256642, the underlying application of which is also assigned to the assignee of the present invention.